Currently, rapidly increasing demand for communication causes a rapid increase in the use of an optical waveguide device for optical communication, optical signal processing, and optical connection. Particularly, the ability to easily arrange optical fibers is needed in order to rapidly transfer an optical signal. To satisfy the above need, the production of a multimode optical waveguide device having a large core layer is considered very important, and, recently, has been frequently studied.
Typically, the optical waveguide device is produced using a semiconductor production technology or a MEMS (micro electro mechanical system) technology, and a planar optical waveguide technology is employed in the course of producing the optical waveguide device on a planar substrate. Additionally, studies have been continuously made to further integrate functions of the optical waveguide device.
A conventional method of producing the optical waveguide device is as follows. After a lower clad layer is formed on a substrate, a core layer is layered on an upper side of the lower clad layer. Subsequently, a photoresist layer is formed on an upper side of the core layer, exposed and developed to form a photoresist pattern. The core layer is etched using the photoresist pattern, thus the core layer is patterned. Next, an upper clad layer is formed on an upper side of the patterned core layer, thereby creating the optical waveguide.
The clad layer or the core layer is typically formed through a spin coating process and a deposition process, and is made of silicas having different refractive indices or polymers having different refractive indices. However, if silica is used as a material for the core and clad layers, the difference between the refractive indices of the core and clad layers is a maximum of 0.75%. Accordingly, use of the above materials is problematic in that it is difficult to produce a device for multimode optical communication because the size of the optical waveguide is limited.
The above method is problematic in that it increases the cost of the optical waveguide device and reduces reliability. Hence, many processes have been suggested to simplify stages constituting the method of producing the optical waveguide. Of them, the simplest process is a technology of forming an optical waveguide, in which a photosensitive material the refractive index and thickness of which have been permanently changed by light radiation is directly photo-patterned on a coat.
U.S. Pat. Nos. 3,809,732 and 3,953,602 disclose a method of producing an optical waveguide, in which a refractive index and a thickness of a polymer material are changed through a photo-locking process. However, the use of only the polymer material leads to many problems in the course of producing a multimode optical waveguide having a large core layer because the material has a low thermal stability, and because light dispersion and loss are large. Furthermore, Korean Patent Registration No. 0426959 discloses a method of producing an optical waveguide through direct photopatterning using an inorganic/organic nano hybrid material, which is produced employing a sol-gel method, and a photosensitive hybrid material, which is doped with a photoinitiator for forming dimers to initiate polymerization. However, it is difficult to produce a thick layer, which is largely shrunken during hardening or photo-patterning, using the photosensitive polymer material and the inorganic/organic nano hybrid material. Thus, it is difficult to produce a multimode optical waveguide which includes a structure having a size of 10 microns or more.
Conventional inorganic/organic nano hybrid polymer is produced through a sol-gel method, in which organometallic alkoxide is hydrolyzed and condensed using water and a catalyst to produce a solution, and is then hardened. U.S. Pat. Nos. 6,054,253, 5,774,603, and 6,309,803 disclose a process of applying an inorganic/organic nano hybrid polymer, which is produced through the above sol-gel method, to an optical device. As well, the present inventors (Bae et al.) have suggested a method of producing an optical waveguide through direct photopatterning using an inorganic/organic nano hybrid polymer produced through a sol-gel method, as disclosed in Journal of Materials Research (JMR), 16[11], pp 3184-3187 (2001).
However, since the inorganic/organic nano hybrid polymer, which is produced through the above method, is insufficiently cured at low temperatures, silanol groups remain in the hybrid material. The remaining silanol groups absorb wavelengths of 1310 nm and 1550 nm, which correspond to near infrared rays currently used in optical communication, thus a transmission loss is undesirably large. Furthermore, if the inorganic/organic nano hybrid polymer is used for a long time, moisture in atmospheric air is adsorbed onto the silanol groups in the material, thus the performance of a device may be reduced. As well, the inorganic/organic nano hybrid polymer, which is produced through the above method, is disadvantageous in that it is difficult to produce the desirable multimode optical waveguide because of a difficulty in forming a film having a size of 10 microns or more, which is considered to be one of the most fatal problems.